Numerous ink systems are available in the marketplace. These systems are often designed for specific utilities.
A fairly recently introduced concept to the packaging industry is the use of retort pouches for medical implements, foodstuffs and the like. These pouches are formed from a pair of lamina. Each lamina comprises a pair of plastics resin film layers sandwiching and adhered to a metallic foil layer, such as aluminum foil. Thus, the contents within the pouch cannot be seen, and suitable labeling is often required.
Many ink systems are available which successfully print on plastics resin films, such as polyester films. However, most ink systems which successfully print on plastic resin film layers cannot withstand the heat of retort sterilization, which normally takes place at approximately 250.degree. F. (121.1.degree. C.) or higher, or if the ink system is able to withstand the retort operation, the ink system blocks severely when a printed roll of film is stored. The term "blocking" refers to the tendency for the ink to adhere to a surface against which it comes into contact, other than the surface it was printed onto. Thus, when a printed roll of film is coiled, blocking refers to the tendency for the ink to adhere to successive layers of the film which have been coiled on top of the printed layer.